Electroplating, because of its inherent simplicity, is used as a manufacturing technique for the fabrication of metal and metal alloy films. One of the severe problems in plating metal films arises from the fact that when a plating current is applied the current tends to spread in the electrolyte on its path from the anode to the cathode. This current spreading leads to non-uniform local current density distribution on the cathode. Thus, the film is deposited in a non-uniform fashion, that is, the thickness of the film varies in direct proportion with the current density variation at the cathode. Additionally, where metal alloy films are deposited, for example, magnetic film compositions of nickel and iron (permalloy) or nickel, iron and copper, this non-uniform current density distribution causes a variation in the composition makeup of the alloy film.
When plating is used for the purpose of making thin film electronic components such as conductors and magnetic devices such as propagation and switch elements, where both thickness and alloy composition determine the operation of the device, the uniformity of thickness and alloy composition are very important and critical. In connection with this, one distinguishes between the variations in composition of the alloy through the thickness of the film and between the variation of composition and/or thickness from spot to spot laterally over the entire plated wafer (cathode).
The patent to Croll et al, U.S. Pat. No. 3,317,410 and the patent to Bond et al, U.S. Pat. No. 3,809,642 use a flow-through anode and an anode housing with a perforate area for increasing the thickness uniformity. The patent to Powers et al, U.S. Pat. No. 3,652,442, improved the thickness uniformity by placing the electrodes in the cell such that their edges are substantially in contact with the insulating walls of the cell. These processes were advances in the state of the art and did improve the uniformity of the plating layer to an extent sufficient for use at that time.
In magnetic bubble modules all of the generator, switches, propagation elements, expander, detector, sensor and the like are made of thin permalloy elements that range in size from &lt;1 micron to over 15 microns. These permalloy elements are made by either a subtractive process or an additive process. The subtractive process involves vapor depositing a layer of permalloy on a substrate and using a photoresist mask to etch the permalloy away leaving the desired permalloy pattern. A minimum gap or part size of the order of 1 micron or less is difficult to obtain due to the control of the line width needed in two processes, photolithography and ion milling. Also, redeposition of permalloy during ion milling degrades the permalloy magnetic properties.
The additive process involves applying a flash coating of permalloy on the substrate followed by depositing a photoresist mask and then plating the desired elements directly on the substrate in the mask openings. The plating directly replicates the photolithography pattern; line and gap control of the permalloy are only influenced by one process, photolithography. With the additive process, gaps or part sizes in the 1 micron or sub-micron range are obtainable. However, for the additive process to be acceptable, it is necessary to have uniform thickness, composition, and magnetic properties in the plated permalloy that have not been obtainable with the prior art plating apparati and methods described above.